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SubjectRe: [PATCH] sched/debug: avoid executing show_state and causing rcu stall warning
On Wed, Aug 03, 2022 at 07:14:52PM +0200, Ingo Molnar wrote:
>
> * Paul E. McKenney <paulmck@kernel.org> wrote:
>
> > There is an rcu_sysrq_start() and rcu_sysrq_end() to suppress this.
> > These are invoked by __handle_sysrq(). The value of
> > rcu_cpu_stall_suppress should be non-zero during the sysrq execution, and
> > this should prevent RCU CPU stall warnings from being printed.
> >
> > That said, the code currently does not support overlapping calls to the
> > various functions that suppress RCU CPU stall warnings. Except that the
> > only other use in current mainline is rcu_panic(), which never
> > unsuppresses.
> >
> > So could you please check the value of rcu_cpu_stall_suppress? Just in
> > case some other form of suppression was added somewhere that I missed?
>
> So instead of supressing the (justified!) RCU stall messages, I'd much
> rather we apply only the minimal locking necessary for this debug printout.
>
> That should also solve the stall warnings as a side effect.

I am certainly with you in spirit! If I recall correctly, the issue
that led to the current state was that there was no way to walk the
task list locklessly except under an RCU read-side critical section.
Yes, you can use get_task_struct(), but that only prevents that task
structure from being freed, not from being removed from the list.

Here is hoping that there is a better way to nail down a task while
RCU-pausing a task-list traversal. Thoughts?

Thanx, Paul

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